An organic electroluminescent device (the “electroluminescent” will be referred to as “EL” hereinafter) has the properties of high visibility based on self-emission, excellent impact resistance based on a complete solid device, and the like, so that it attracts attention for its use as an emitting device in various displays.
The organic EL device has an emitting layer interposed between a pair of electrodes consisting of an anode and a cathode. When an electric field is applied between these two electrodes, electrons are injected into the emitting layer from the cathode side, holes are injected into the emitting layer from the anode side, and the electrons and the holes are recombined in the emitting layer to emit light.
As a technique of increasing the efficiency of the organic EL device, there are the technique of doping the emitting layer and the technique of improving a cathode material, and attempts have been made to improve the organic EL device in current efficiency.
On the other hand, a voltage-decreasing technique has been developed for decreasing the power consumption, and a remarkable decrease in the voltage for an organic EL device has been confirmed when an organic material having high mobility of electrons or holes is used.
However, it cannot be easily attained to realize both high current efficiency and a decrease in voltage at the same time. Further, it is very difficult to attain higher lifetime simultaneously.
In particular, an organic EL device for emitting blue light has the above difficulties to a great extent, and various improvements have been carried out, but they cannot be said to be sufficient.
As a technique for higher efficiency and a decrease in voltage, the use of a hole-blocking layer is proposed (e.g., JP-A-2-195683). The hole-blocking layer refers to a layer provided for preventing holes injected into the emitting layer from being diffused in the cathode direction. However, when an emitting layer has a wider gap like blue emission, the ionization potential is large, so that a hole-blocking layer to be combined therewith is required to have a further larger ionization potential. As a result, it is difficult to select a material therefor, and the lifetime of such a layer is often short, so that the hole-blocking layer has not been put to practical use.
For adjusting the ratio of hole blocking, it is required to select different hole-blocking materials, and when emitting layers differ, it is required to select hole-blocking materials to match them, respectively.
Meanwhile, as an electron injecting layer, there are reports on those which have remarkably high electron mobility, and it is confirmed that the use thereof enables an organic EL device to perform at a remarkably decreased voltage (for example, see JP-A-9-087616 and JP-A-9-194487). In this case, however, the quantity of electrons injected into the emitting layer comes to be too large as compared with the quantity of holes, so that electrons pass through the emitting layer at a high ratio in many cases. Therefore, a large amount of the current flows since the current is an addition of flows of holes and electrons. However, the emission amount is not so large. That is, the problem is that no sufficient emission efficiency can be obtained, and there is also involved another problem that the lifetime is short.
In view of these problems, it is an object of the invention to provide an organic EL device improved in emission efficiency.
In particular, it is an object of the invention to provide an organic EL device having high emission efficiency at a low voltage.